With development of display technologies, images are displayed in increasingly varied ways. The illumination of the displayed image can be changed. As an image consists of pixels, and a grey scale of a pixel represents an illumination level of the pixel, a terminal device can display a pixel in an image at a certain grey scale so as to control the illumination of the image.
However, when the environment becomes dark, the image displayed by the terminal device is usually striking. To protect the user's eyes, it needs to reduce the grey scales of the pixels in the image to thereby reduce the illumination of the image.
However, the contrast of the image is determined by the difference between different pixel grey scales. When the terminal device reduces the pixel grey scales of the image, the difference between different pixel grey scales is also reduced. As a result, the contrast of the image is reduced, and the definition level of the image is affected.